The present invention relates to a bolt having an excellent guiding performance.
Bolts have been known which are provided at a tip end of a bolt shank portion with a guide portion so that, even when a bolt is inserted slantingly into a mating female screw, the bolt corrects its own position to be precisely inserted into the female screw (as disclosed in, for example, Japanese Provisional Utility Model Publications Nos. 5-57415, 59-37413, 54-147364, 6-59614, 52-77371 and 50-79861). FIG. 3 shows an example of prior bolts devised in that manner, in which a tapered portion 3 is provided on a tip end of a bolt shank portion 2 formed with normal screw threads 1 and screw threads 4 are also formed on the tapered portion.
It is believed that while the tapered portion 3 is sometimes formed to define a simply tapered slant face, it can be formed with the screw threads 4 whereby, even when an associated bolt is inserted somewhat slantingly into a mating female screw such as nuts or the like, the screw threads formed on the tapered portion engage with screw threads 6 of the female screw to enable guiding the bolt precisely relative to the female screw while correcting a position of the bolt.
However, it is not possible to form complete screw threads on the tapered portion 3, and so the screw threads 4 on such portion become necessarily incomplete. Such incomplete screw threads are made smaller in height than complete screw threads are. Therefore, when a slant angle .theta. (see FIG. 4. The same applies in the following description) of a central axis of the bolt relative to a central axis of a female screw 5 is small, the bolt can be corrected with respect to a direction of insertion, but when a slant angle .theta. is great, the screw threads on the tapered portion will not engage with the screw threads 6 on the female screw 5, and so undergo slipping to crush the screw threads on the female screw and to generate pitch jumping with the result that continued fastening causes a danger of seizure on the bolt.
Further, in the case where such bolts are fastened by means of an automatic fastening machine, not only the slant angle but also axial misalignment (misalignment between the bolt axis and the female screw axis) become problematic. Incidentally, an example of the user's demands is that the slant angle .theta. is at least 3 degrees with the axial misalignment being 3 mm.